The present invention relates to a displacer unit, and, in particular, a piston pump with a plurality of displacers or pistons which carry out sequential displacer or piston working strokes.
In fluid and hydraulic systems of motor vehicles, pumps piston, which are usually configured as radial piston pumps, are used for supplying different consumption units. In addition to being functionally reliable, it is also important that the pump should operate with low noise in order to permit comfortable driving in the vehicle.
To avoid noise, account must be taken of the fact that the pump must operate at greatly varying speeds depending on the operating conditions of the vehicle and the speed of the vehicle engine and, as far as possible, low-noise operation should be possible at all speed ranges.
In connection with the reduction of operating noise in a vehicle, it is known in principle to encapsulate units causing noise in a sound insulating manner or to arrange them in such a way that the noise cannot be transmitted, or can only be transmitted to a small proportion, to other components or sub-assemblies of the vehicle.
Nevertheless, it remains a desirable target to employ, as far as possible, such units as produce particularly low noise so that the measures mentioned above become unnecessary to the greatest possible extent.
A radial piston pump with three pistons is shown in DE-B 12 07 707 which is actuated by a cam ring with five cam-shaped protrusions. Two of the pistons have the same cross-sections and are located with their axes at an angle of approximately 72.degree. to one another. The third piston has approximately 1.6 times the cross-section of the two other pistons and is located with its axis symmetrical to the piston axes of the two other pistons. Because of this arrangement, all the pistons execute their compression or suction stroke simultaneously, i.e. all the pistons are simultaneously moved radially inwards or radially outwards. Because of the arrangement and the dimensions of the pistons, the reaction torques occurring are mutually compensating, i.e. the housing of this known pump is only excited to relatively small vibrations by the pump work.
Another radial piston pump is known from U.S. Pat. No. 2,423,701 whose pistons are actuated by a central cam wheel with three cams. Different piston arrangements are possible depending on the desired pump output. In one embodiment, two diametrically opposite and coaxial pistons are associated with the cam wheel. In another embodiment, the cam wheel actuates a total of four pistons which are opposite to one another in pairs, with the pistons of each pair having a common piston axis. The piston axes of pistons adjacent to one another in the direction of rotation of the cam wheel form a right angle in each case. Finally, a third embodiment is shown in which the cam wheel interacts with three pistons, of which two coaxial pistons are diametrically opposite to one another. The third piston is arranged transverse to the two coaxial pistons and its piston axis forms a right angle with each of the piston axes of the two other pistons.
Even though the previously known piston pumps operate, in some cases, with very little vibration, undesirable noise still appears during operation, particularly in motor vehicles.
An object of the present invention is, therefore, to produce a construction which leads to a substantial reduction in noise. This object has been achieved, according to the present invention in a displacer unit or a piston pump in which there are varying phase differences between working strokes occurring sequentially in time, with these phase differences having a non-integral ratio with one another.
The present invention is based on the recognition that an essential cause of the noise developed by displacer units and piston pumps is that pressure pulsations necessarily appear in the pipework system connected to the displacer unit or piston pump. In the case of piston pumps, this applies particularly to the pressure pulsations on the pressure side of the pump. These pressure pulsations can lead to noise at very remote locations, particularly when natural frequencies are excited there of vibrating components or units. In motor vehicle, for examples, body parts can be excited to vibration relatively easily even at a large distance from a hydraulic pump if these body parts are located near fluid pipes in which pressure pulsations can occur at a critical frequency. An additional difficulty is that in motor vehicles, the pumps are usually driven directly by the vehicle engine and therefore run with varying speed depending on the particular vehicle speed. The risk that the pressure pulsations caused by the pump may have a critical frequency for some parts of the body, at least in certain speed ranges, is very great.
Because, in accordance with the present invention, there are different time intervals between sequential working strokes of the displacer unit or the piston pump, and since the length of these time intervals have a non-integral ratio with one another, this arrangement at least avoids the possibility of the pressure pulsations having a frequency spectrum in which the frequencies of higher harmonics are excited in addition to the fundamental vibration frequency. What is, in fact, excited is a mixture of frequencies whose constituents cannot mutually amplify one another so that the formation of resonances is quite substantially limited.
The invention can be fundamentally effected in the case of very different types of displacer unit or piston pump configurations. In a piston pump with cylinders arranged in line, for example, a crankshaft can be provided whose cranks can form appropriately different angles, in an axial view onto the shaft, so that the piston strokes follow sequentially with varying phase differences.
Pistons or displacers driven by eccentric or cam units can have a multiplicity of eccentric regions or cams which are so arranged that, for a constant driving speed, they become effective as driving elements with different time intervals between them.
In a particularly preferred embodiment of the present invention, a piston pump is configured as a radial piston pump, with at least some of the cylinders thereof associated with the pistons being arranged at different angles relative to one another so that there are correspondingly varying phase differences between working strokes which are sequential in time.